Process water chemistry in bitumen extraction from oil sands

ABSTRACT

Methods of balancing, monitoring and maintaining process water chemistry within predetermined limits to enhance bitumen extraction and recovery from oil sands ore.

FIELD OF THE INVENTION

This invention relates to methods of balancing, monitoring and maintaining process water chemistry within predetermined limits to enhance bitumen extraction and recovery from oil sands ore.

BACKGROUND

Oil sands ore, known as bituminous sands, comprises a naturally occurring mixture of water, bitumen and a mineral phase comprising sand and clay. Oil sands ore may vary in quality and character from region to region, or deposit to deposit. Generally, Athabasca oil sands in Canada comprises water-wet sand grains, while Utah oil sands in the United States comprises oil-wet sand grains. The sand grains themselves may be of different composition. The mineral phase of Utah oil sands may be characterized by carbonate cementation, where upwards of 15 wt % of the mineral phase is calcite and dolomite.

Oil sand extraction processes are used to liberate and separate bitumen from the water and mineral phases such that the bitumen can be further processed to produce synthetic crude oil. Numerous oil sands mining and bitumen extraction processes have been developed and commercialized, all of which involve the use of water as a processing medium. One such water extraction process is the Clark hot water extraction process, which was the first commercially successful oil sand extraction process.

A water extraction process such as the Clark process typically requires that mined oil sand be conditioned for extraction by being crushed to a desired lump size and then combined with water and caustic to form a conditioned slurry of bitumen, water , sand and fine particles. In the Clark process, the water used is heated to about 65° to 80° Celsius, and an amount of sodium hydroxide (caustic) is added to the slurry to adjust the slurry pH upwards, which enhances the separation of bitumen from the oil sand. The addition of caustic is intended to elevate the concentration of natural surfactants through an acid-base reaction with organic acids present in the bitumen, and to increase the softness of the water phase by increasing the concentration of sodium ions. Other water extraction processes may have other temperature requirements and may include other conditioning agents which are added to the oil sand slurry.

A bitumen extraction process will typically result in the production of a number of product streams, some of which are disposed of as waste. For example, in the Clark process, these streams include a bitumen froth stream comprising of aerated bitumen, fine particulate mineral solids and water, a middlings stream comprising bitumen, fine particulate mineral solids and water, and a coarse tailings stream consisting primarily of coarse particulate mineral solids and water. The bitumen froth stream and the middlings stream are typically processed further, both to recover and purify bitumen and to render the fine solids more readily disposable and less of an environmental hazard. The coarse tailings stream is not typically processed further, since the coarse particulate solids are relatively easy to dispose of and do not typically present a significant environmental risk.

The bitumen froth stream is processed in a froth treatment to deaerate, separate water and fine solids. The fine solids and water recovered from the bitumen froth stream are typically ultimately disposed of in tailings ponds, where subsequent consolidation of the fine solids occurs and the water recovered and reused.

The relative levels of sodium (Na) to calcium (Ca) cations, and pH are well known process chemistry variables in oil separation, surfactant concentration and performance. These parameters play a role in modulating the strength of the attraction between mineral particle and oil droplet surfaces and in determining the affinity of oil droplets for the process water by influencing the presence of and activity of natural surfactants derived from organic acids in the oil phase. These principles are well known in Athabasca oil sands bitumen recovery performance and in determining the quality of the final product.

The use of basic sodium salts to maintain water softness and alkalinity in oil recovery and degreasing processes utilizing detergents or surfactants is pervasive. Additionally, it is becoming more important in Athabasca oil sands mining operations to reuse and recycle process water to minimize fresh water usage, and higher water recycle efficiencies and reduced water use result will result in accelerated changes in process water chemistry. This is especially true for oil sands ore that contain significant sources of water soluble hard cations such as calcium (Ca), as is commonly found in carbonate cemented sandstone oil sands ore from Utah.

Accordingly, there remains a need in the art for methods of controlling process water chemistry which may improve bitumen extraction efficiency.

SUMMARY OF THE INVENTION

Aspects of this invention are directed to a water chemistry control system that is intended to facilitate maintaining high process availability at optimal water chemistry conditions in oil sands extraction facilities that are increasingly utilizing higher water turnover rates through the plant and supporting water containment facilities.

The first stage of the process water chemistry control is to keep water softness (Na:hard cations ratio) and alkalinity (pH) within a desired range during ore digestion, primary extraction and secondary extraction, by adding a sodium salt as needed. The specific mechanism associated with this surfactant generation reaction is thought to be the base neutralization of organic acids (e.g., ROOH) present in the oil phase, resulting in the formation of dissolved natural surfactants (e.g., Na⁺—OO—R) that aid in stabilizing oil separation from mineral surfaces.

The role of water softness (e.g., the ratio of [Na] to [Ca] and other hard cations) is likewise important due to the competing equilibrium reaction between labile sodium-associated organic surfactants (e.g., Na⁺—OO—R) and polyvalent cation-associated organic surfactants that precipitate out of solution as organic solids. This competition may result in the reduction of viable dissolved surfactants, which are required to effectively stabilize oil droplets in the water phase (i.e., impeding oil-mineral separation).

It is known that tight oil-in-water emulsions form at higher pH and elevated softness states, which can reduce recovery in secondary extraction. For example, a tight emulsion may result in the inability of a disk stack centrifuge to separate the oil and water feed because the oil phase is too finely dispersed in the water phase. Therefore, a secondary process control is required downstream in or after secondary extraction to act to maintain the [Na⁺] to [Ca²⁺] ratio (i.e., water softness) and pH through the addition of a calcium salt solution or other polyvalent cation source, such as acid alum.

The addition of a calcium salt may keep pH and the [Na] to [Ca] ratio from rising above certain upper limits. The specific reactions that this second stage of control affords may allow oil sands mining operators to reduce process water usage and therefore eliminate or minimize the use of tailings ponds.

Furthermore, this secondary control may aid in limiting increase in concentration of organic anions (i.e., surfactants) that can also result in heat exchanger fouling, as is commonly observed in Steam Addition Gravity Drainage (SAGD) facilities. It is commonly found that pH in oil sands mining-extraction facilities varies between 8.5 to 9.0, the [Na] to [Ca] ratio between 400 to 600 and dissolved organic anions (surfactants) between 30 to 100 mg/L.

The addition of calcium may also reduce process water pH by consuming hydroxide anions (OH⁻), resulting in the precipitation of calcium hydroxide (Ca(OH)₂, if free calcium cations are added at a rate that exceeds its solubility limit. Once the process pH adjusts to a new equilibrium level that matches the dosage rate of the calcium salt, the free calcium concentration will rise and therefore the [Na] to [Ca] ratio will fall. Finally, calcium salt addition may also result in the favorable equilibrium displacement of Na⁺—OO—R (surfactants) towards the production of polyvalent cation-associated organic surfactants that precipitate out of solution as organic solids.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the specification and are included to further demonstrate certain embodiments or various aspects of the invention. In some instances, embodiments of the invention can be best understood by referring to the accompanying drawings in combination with the detailed description presented herein. The description and accompanying drawings may highlight a certain specific example, or a certain aspect of the invention. However, one skilled in the art will understand that portions of the example or aspect may be used in combination with other examples or aspects of the invention.

FIG. 1 shows a process flow diagram of one embodiment of a method of the present invention.

FIG. 2 is a schematic representation of the primary and secondary extraction stages of one embodiment of the present invention.

FIG. 3 is an illustration of process water with and without calcium salt addition.

DETAILED DESCRIPTION

As used herein, certain terms have the meanings defined below. All other terms and phrases used in this specification have their ordinary meanings as one of skilled in the art would understand.

In one example, the present invention relates to an improvement to a process of extracting bitumen from oil sands ore using a biosolvent, such as a terpene, and water. Generally, the method is applied to oil sands ore mined or produced conventionally, and initially processed by contacting the ore with a solvent, such as a biosolvent, and water and creating an aqueous emulsion of diluted bitumen. The general steps of this method are described in co-owned U.S. Pat. No. 8,758,601 and co-pending U.S. patent application Ser. No. 14/959,910—Oilsands Processing Using Inline Agitation and an Inclined Plate Separator, filed Dec. 4, 2015, the entire contents of both which are incorporated herein by reference, where permitted.

In one embodiment, the methods described herein contemplate the use of a biosolvent such as terpenes. Terpenes may be acidic, which may necessitate the use of a basic sodium salt to neutralize organic acids and to limit solubility of components of the mineral phase which may hinder bitumen extraction.

The solvent/water/oil sands mixture first passes through a rotating screen or trommel (10), which allows smaller solids and liquid to pass through, while coarser solids are retained on the screen and disposed of. This fine solids and liquid mixture is then treated in a primary extraction stage which takes place in a primary separation vessel or PSV (12). In the PSV (12), bitumen diluted by the solvent is liberated from the mineral phase, and gravity separation of any remaining coarse solids takes place. The overflow from the PSV, which comprises diluted bitumen, water and fine solids, is then directed downstream for further treatment. In one embodiment, two PSVs are used in series, where the second PSV (12B) accepts the underflow from the first PSV (12A), The overflow from both PSVs are combined and further treated in a secondary extraction stage. The underflow from the second PSV (12B) consists primarily of process water and coarse solids and is directed to a screen shaker to separate the solids, and recover the process water for reuse. The process water recovered from the second PSV underflow may tested and treated to maintain pH and sodium/calcium ratios as described herein. It is likely that this process water will require some calcium salt addition, shown as “calcium salt addition pt. 2” in FIG. 1.

In one aspect, a caustic sodium salt is first added as the solvent/water/oil sands are first mixed or before the first PSV (12A). In one embodiment, the caustic sodium salt may comprise sodium silicate or sodium hydroxide, and is added in an amount necessary to achieve the desired pH and water softness. In one embodiment, water softness is measured by the molar sodium/calcium ratio of the water. In one embodiment, pH of the process water in the primary extraction stage is in the range of about 9 to about 9.5, and the molar sodium/calcium ratio is between about 100:1 to about 200:1, and preferably between about 125:1 to about 150:1. Caustic sodium salts affect the sodium/calcium balance in part by directly adding sodium to the mixture, and in part by limiting the solubility of calcium from the mineral phase, which itself is determined in part by the grade of ore and/or the degree of carbonate cementation.

A similar rationale can apply to the use of acidified alum solutions in place of calcium salts. Acid alum solutions are used in water treatment of dissolved organics and suspended solids and the physical mechanisms that apply in respect of calcium cations described above also apply to aluminum as a polyvalent cation. In general, the goal is to return the process water to a “harder” state, after “softening” the water with the addition of sodium. Polyvalent cations such as calcium, magnesium, aluminum are typically responsible for water hardness.

The initial pH level and sodium/calcium ratio may be immediately measured from the mixture first created and subsequently adjusted by addition of chemicals at any point prior to or during primary extraction. Process water chemistry may then be monitored and maintained through the primary extraction stage.

For example, in one embodiment, the pH level and/or sodium/calcium ratio may then be measured again in the underflow (14) of the first PSV (12A), and adjusted by the further addition, if necessary, of a caustic sodium salt before or in the second PSV (12B). The goal of optimizing the pH and sodium/calcium balance in the primary extraction stage is to maximize separation of the bitumen from the mineral phase.

The overflow (16) from the primary extraction stage comprises the bulk of the desired bitumen product, which must now be recovered. A secondary extraction stage treats the combined PSV overflow (16) and, in one embodiment, incorporates an agitator or inline mixer (18) and a separator (20). The combined PSV overflow may comprise approximately 90% water by volume, with the balance roughly equally split between diluted bitumen and solids. This mixture may be separated using centrifuges, however, it is desirable to further concentrate the diluted bitumen before a centrifuge stage. Therefore, in one embodiment, this stream is agitated or sheared in the inline mixer (18) to produce a very fine emulsion of the diluted bitumen in water. Without restriction to a theory, it is believed that the diluted bitumen associates with the solids during the shear agitation step as a result of the fine solids migrating to the oil/water interface of the emulsion during the agitation step, possibly via calcium or other polyvalent cations presented by carbonate minerals present in the solids, forming a hydrocarbon-mineral complex. As such, this method may be particularly suitable for use with carbonate cemented oil sands, or oil-wet oil sands, such as those found in Utah, New Mexico or California. The specific gravity of hydrocarbon-mineral complexes is sufficiently high to cause those complexes to settle and be removed via the underflow of a separator (20).

In one embodiment, the agitation comprises a shearing action to micronize the oil-in-water emulsion, where the diluted bitumen is an emulsified into sufficiently small and disperse droplets such that a majority of the bitumen associates with the mineral phase, and separates from the water phase. In one embodiment, the oil phase droplets are, on average, less than about 100 μm, preferably less than about 50 μm, and more preferably less than about 30 μm. In one preferred embodiment, a majority of the oil droplets may be in the range of about 10 μm.

The agitator (18) may comprise any apparatus capable of mixing the PSV overflow to produce the desired micronized emulsion, such as a blender, jet agitator, or the like. In one embodiment, the agitator may comprise an inline mixer (18) comprising of a pipe defining a plurality of openings, such as a slotted pipe, through which the PSV overflow is forced through under pressure. As a result, the overflow stream passes through the openings with sufficient velocity to induce a shearing action within the fluid.

In one embodiment, the micronized emulsion is treated in an inclined plate separator (20) or IPS to produce the bitumen stream in the IPS underflow (22), and the water stream in the IPS overflow (24). In one embodiment, the overflow (21) from a primary separator (20A) may be passed to a secondary separator (20B).

A substantial portion of the diluted bitumen will be in the IPS underflow (22), which will then comprise less water on both a volume % and flow rate basis than the overflow from the PSV. For example, if the PSV overflow (16) comprises about 90% water (vol.), then the IPS underflow (22) may comprise about 60 to 70% water, while the IPS overflow (24) may comprise about 98 to 99% water.

The IPS overflow (21) from the primary separator (20A) may be monitored for pH level and/or sodium/calcium ratio, which may be adjusted by the further addition, if necessary, of a caustic sodium salt. The goal of optimizing the pH and salt balance in the secondary separator (20B) is to maximize recovery of bitumen.

The underflow (22) from both the primary and secondary separators is combined and processed in a centrifuge or decanter (26), which produces separate fine solids, water and diluted bitumen streams. The water stream and diluted bitumen flows to the disk stack (28) for final polishing and from the disk stack, process water is returned to bulk water storage (recycled). The diluted bitumen product is then recovered, substantially free of water and solids.

In one embodiment, it is preferred to monitor and maintain the pH and sodium/calcium ratio in the process water as it is recovered from the centrifuges (26), before recycling to the primary extraction stage. It is likely at this stage that the pH and sodium/calcium ratio has exceeded preferred parameters. Therefore, at this stage, in one embodiment, it is possible to add an acidifying agent and/or calcium salts to the process water in order to reduce the pH and/or the sodium/calcium ratio. In one embodiment, the acidifying agent may comprise a mineral or organic acid, or may comprise carbon dioxide which produces carbonic acid upon dissolution in water. In one embodiment, the calcium salt may comprise CaCl₂ or CaSO₄.

As shown in FIG. 1, the acidifying agent and/or calcium salt may be added to the water stream recovered from the centrifuge (24) stage, or to a combined process water stream which combines the water stream from the centrifuges, the oil/water separator or OWS (26) and the screen shaker. Fresh makeup water may be added at this point as the process water is routed back to the ore crushing and solvent mixing stage,

Each of the chemical process aid addition points may coincide with, or be slightly downstream from a parameter monitoring point. In one embodiment, the monitoring of the process water and dosing of the process aids may be automated, using primary process logic controls allowing for variable dosage delivery as a function of process parameters known to those skilled in the art.

The effect of calcium salt addition to the process water recovered from the IPS overflow may be seen in FIG. 3. The first two jars are taken from the IPS overflow without (Jar A) and with (Jar B) calcium salt addition. Suspended solids and dispersed oil content is decreased with calcium salt.

In addition, the IPS overflow stream returns back to the bulk process water tank corrected in terms of pH and hardness such that neither parameter can cycle upwards as the process water is reused for primary extraction. Jars C and D were taken at the same time intervals, but are from the front end of the plant coming out of the ore digestion or trommel unit in water circulation mode, with no ore added. Jar D is with calcium salt addition, and it may be seen from the lighter color that there is less dispersed oil in the water. This overall test was done on dirty process water, without ore addition so as to obtain a first order estimate on the dual stage dosage rates required to balance the system as shown schematically in FIG. 2.

Definitions and Interpretation

The description of the present invention has been presented for purposes of illustration and description, but it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. To the extent that the following description is of a specific embodiment or a particular use of the invention, it is intended to be illustrative only, and not limiting of the claimed invention.

The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims appended to this specification are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.

References in the specification to “one embodiment”, “an embodiment”, etc., indicate that the embodiment described may include a particular aspect, feature, structure, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to combine, affect or connect such aspect, feature, structure, or characteristic with other embodiments, whether or not such connection or combination is explicitly described. In other words, any element or feature may be combined with any other element or feature in different embodiments, unless there is an obvious or inherent incompatibility between the two, or it is specifically excluded.

It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as “solely,” “only,” and the like, in connection with the recitation of claim elements or use of a “negative” limitation. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

The singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. The term “and/or” means any one of the items, any combination of the items, or all of the items with which this term is associated.

As will also be understood by one skilled in the art, all ranges described herein, and all language such as “up to”, “at least”, “greater than”, “less than”, “more than”, “or more”, and the like, include the number(s) recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. 

What is claimed is:
 1. A method of monitoring and maintaining process water chemistry in a bitumen extraction process including a primary extraction stage including at least one primary separation vessel, and a secondary extraction stage comprising at least one separator, comprising the steps of: (a) adding a caustic sodium salt to a mixture of oil sands, solvent and water to create a slurry comprising process water having a pH of about 9.0 to about 9.5, and a molar ratio of sodium to calcium ions between about 100:1 to about 200:1; (b) maintaining the process water pH and sodium to calcium ratio in the ranges described in step (a) through the primary extraction stage, the secondary extraction stage and before recycling for reuse, by measuring the process water chemistry and adding chemicals as needed.
 2. The method of claim 1 comprising the step of adding an acidifying agent or a calcium salt, or both, after the secondary extraction stage to maintain the process water pH and sodium to calcium ratio in the desired ranges.
 3. The method of claim 2 comprising the further step of recycling the process water to the primary extraction stage.
 4. The method of claim 1 wherein the molar ratio of sodium to calcium is maintained between about 125:1 to about 150:1.
 5. The method of claim 4 wherein the secondary extraction stage comprises an agitation step and a separation step, whereby diluted bitumen is recovered associated with fine solids from the separation step.
 6. A system for extracting bitumen from oil sands, comprising a primary extraction stage including at least one primary separation vessel; a secondary extraction stage comprising at least one separator; at least one caustic sodium salt addition point in the primary extraction stage; and at least one calcium or polyvalent cation addition point in the secondary extraction stage. 